The present invention relates to biodegradable starch acetate polymers, blends, and compositions, and methods for making them.
The term "biodegradable" does not yet have a generally accepted meaning in the plastics industry. Agencies such as the Food & Drug Administration ("FDA") and the Environmental Protection Agency ("EPA") have not, to date, promulgated a test for establishing which products are "biodegradable." In general, the term biodegradable has been applied to any material which is meant to decompose significantly when placed in land fills. Unfortunately, many plastic formulations are said to be biodegradable even when composed of mostly nonbiodegradable oil-based polymers. For example, a formulation of 98% by weight polyethylene and 2% by weight corn starch is called "biodegradable" because over time the corn starch binder will decompose and cause the material to break into smaller pieces or chunks of polyethylene. Unfortunately, the resultant pieces of polyethylene will not biodegrade further. Thus, truly biodegradable plastics that safely decompose into primarily carbon dioxide and water are needed.
The use of oil and hydrocarbon gases as the raw material for plastics has dominated the industry. Substantial amounts of carbon dioxide and other toxic gaseous pollutants are released into the atmosphere during the processing to make these raw materials as well as the ultimate plastic products Further, decomposition by-products of oil and hydrocarbon gas based plastics sometimes contaminate ground water. Thus, environmentally safe plastics and processes for making them from benign starting materials are needed.
Plant-derived products are appropriate starting materials for the desired biodegradable plastics. Agricultural plants and their by-products absorb large amounts of carbon dioxide and release large amounts of oxygen during growth. When decomposed, most of the carbon dioxide and water will be recycled to the earth and atmosphere. Starch based polymers such as starch acetate will biodegrade completely and can be made from natural plant-derived materials. However, their potential as environmentally sound commercial materials has not, until now, been realized.
Earlier attempts to make starch-based plastics employed purified starches (usually corn starch) rather than unprocessed flour Purified starches were preferred because they generally produced whiter plastics having broad market appeal. In some cases, whiteness was enhanced by bleaching flour starting materials with sodium hypochlorite or other agents. Purified starches were also preferred because they do not contain simple or complex sugars such as glucose, sucrose, fructose and, in the case of sweet potatoes, maltose. Under conditions previously employed to make starch acetate, these sugars became dark, turning to a sticky char which spoiled the starch acetate product. Thus, pure starch was not only desirable but required to make useful starch acetate.
Unfortunately, additional processing is required to obtain purified starch from flour. Further, the native starch granule size and hence the number of monomer units in each starch molecule decreases when flour is converted to starch. Thus, polymers made from purified starch generally have low molecular weights and tensile strengths. They have not been suitable for consumer products such as containers or wrappings.
In many prior starch acetate synthesis processes, starch is initially dissolved in a low pH acid solution. The heat given off by this exothermic reaction is so great that a low boiling point solvent is often used to prevent a run away reaction. These solvents may be carcinogenic or otherwise hazardous to use in a manufacturing facility. From a practical standpoint, solvents increase the cost of the overall process and thus the price of the final plastic products. Further, if the reactor temperature exceeds an optimum value for too long, low molecular weight plastics will be formed. Charring and/or decomposition of the raw material are also possible if temperatures remain high for too long.